The invention is bearing assembly useful in tools for deployment in wells (i.e., in a downhole environment). One such tool is a drilling assembly including a mud motor power section (e.g., a positive displacement motor or "PDM"), a drill bit assembly, and a bearing section connected between the drill bit assembly and the power section. A PDM is a pump through which fluid (e.g., drilling mud, water, or air) is pumped to rotate an internal rotor within a stator housing. The rotor typically undergoes eccentric motion within the stator housing, and thus a universal joint is usually coupled between the rotor and the bearing section. In response to the eccentrically rotating rotor, the universal joint causes a portion of the bearing section to rotate concentrically within a housing (typically, a stationary tubular member). The drill bit rotates as a unit with the rotating portion of the bearing section.
The bearing section is attached between the drill bit and the PDM along the longitudinal axis of the tool, and the tool is disposed in a well whose central longitudinal axis is substantially parallel to the tool's longitudinal axis. It is desirable that the bearing section accommodate three types of loads: radial loads exerted (perpendicular to the tool's longitudinal axis) between the bearing section's inner rotating portion and the bearing section's outer non-rotating portion; tensile axial loads (equivalent to a force in the uphole direction on the outer non-rotating portion with a force in the downhole direction on the inner rotating portion); and compressive axial loads (equivalent to a force in the downhole direction on the outer non-rotating portion with a force in the uphole direction on the inner rotating portion). The bearing section experiences tensile axial loads, for example, when the tool is off the bottom of the well but the inner rotating section translates downhole toward the bottom of the well. The bearing section experiences compressive axial loads when the drill bit rests on the bottom of the well and rotates against the subterranean formation in response to rotation of the bearing section's inner rotating portion.
In one type of conventional bearing assembly for use in a downhole environment, two axially separated thrust bearings are used: one to accommodate tensile axial loads; the other to accommodate compressive axial loads. However, such a bearing assembly has a long overall length, which limits the curvature of a hole that can be drilled using it (in a directional drilling operation). In order to drill a hole having a short radius of curvature, it is necessary to use a drilling tool having a shorter overall length than can be achieved using a bearing assembly including two such axially separated thrust bearings.
Another type of conventional bearing assembly for use in a downhole environment is described in U.S. Pat. No. 4,511,193, issued Apr. 16, 1985. The bearing assembly of U.S. Pat. No. 4,511,193 includes two bearing sets (84 and 86), each including ball bearings 87 in a race defined by four race elements 88, 100, 110, and 118. Each of race elements 100 and 118 has grooves and lands which lock it to an inner rotatable member (so that elements 100 and 118 rotate as a unit with inner tubular member 26). Each of race elements 88 and 110 has grooves and lands which lock it to an outer stationary member (so that elements 88 and 110 remain stationary with outer stationary tubular member 12, even while member 26 rotates relative to member 12). Each of the ball bearings 87 bears radial, axial tensile, and axial compressive loads. The bearing assembly of U.S. Pat. No. 4,511,193 includes Bellville springs 160, 162, 164, 166, 168, and 170 which maintain an axial preload on bearings 87. Thus, the bearing assembly of U.S. Pat. No. 4,511,193 has a complicated design requiring race elements specially designed to have curved surfaces and to be locked with other components of the assembly, and also requiring axial preloading springs for applying preloading force to ball bearings.
U.S. Pat. No. 5,150,972, issued Sep. 29, 1992, discloses another type of bearing assembly for use downhole, for enabling relative rotation of inner tubular member 14 which is telescopically received in outer tubular member 12 (and including one or more thrust bearings 18 each of which bears axial tensile and axial compressive loads). In the bearing assembly of U.S. Pat. No. 5,150,972, outer tubular member 12 has opposed parallel shoulders 20 and 22, and inner tubular member 14 has opposed parallel shoulders 26 and 28. Shoulders 20, 22, 26, and 28 define a containment chamber. At least one thrust bearing 18 is disposed in the containment chamber such that, when the assembly is placed in compression, shoulder 20 on member 12 bears against a first race of thrust bearing 18 (through spacer 38) and shoulder 28 on member 14 bears against a second race of thrust bearing 18 (through spacer 40). When the assembly is placed in tension, shoulder 22 on member 12 bears against the second race (by bearing on spacer 40) and shoulder 26 on member 14 bears against the first race (through spacer 38). The bearing assembly of U.S. Pat. No. 5,150,972 also includes biasing springs 42 in the containment chamber for maintaining an axial preload on thrust bearing 18.
When the bearing assembly of U.S. Pat. No. 5,150,972 is placed in tension (when running off the bottom of a well), member 14, shoulder 26, spacer 38, and thrust bearing 18 (including race element 34) rotate together as a unit about a longitudinal axis (the horizontal axis of FIGS. 1 and 2 of U.S. Pat. No. 5,150,972) relative to stationary member 12 (including shoulder 22) and spacer 40 which also remains stationary. Because shoulders 20 and 26 and spacer 38 all have load bearing surfaces oriented perpendicular to the longitudinal axis, one or both of elements 38 and 18 will slide transversely against member 12, and/or spacer 40 will slide transversely against member 14, and the frictional force between member 12 and rotating spacer 38 (and/or bearing 18) and/or the frictional force between spacer 40 and member 14 causes substantial heat build up, and ultimately bearing failure. Similarly, when the bearing assembly of U.S. Pat. No. 5,150,972 is placed under axial compression (when running on the bottom of a well), member 14 (including shoulder 28), spacer 40, and thrust bearing 18 (including race element 36) rotate together as a unit about the longitudinal axis relative to stationary member 12 (including shoulder 20) and spacer 38 which remains stationary. Because shoulders 20 and 26 and spacer 38 all have load bearing surfaces oriented perpendicular to the longitudinal axis, one or both of elements 40 and 18 will slide transversely against member 12, and/or spacer 38 will slide transversely against member 14, and the frictional force between member 12 and rotating spacer 40 (and/or bearing 18) and/or the frictional force between spacer 38 and member 14 causes substantial heat build up, and ultimately bearing failure. Thus, the bearing assembly of U.S. Pat. No. 5,150,972 has a design which subjects it to substantial frictional force (and thus heat build up and wear) during operation.
It had not been known until the present invention how to design a downhole bearing apparatus in a manner overcoming the noted disadvantages and limitations of conventional apparatus.